Patent GB-670391-A (GB)
Title: Improvements in or relating to apparatus for the measurement of phase distortion in electrical circuits
IPC: H04B3/46
Filed: 1949-12-21
Abstract: 670,391. Phase difference measurements. GENERAL ELECTRIC CO., Ltd., ESPLEY, D. C., and GLEDHILL, G. A. Dec. 8, 1950 [Dec. 21, 1949], No. 32765/49. Class 37. In order to measure the group delay in a circuit over a band of frequencies, oscillations of frequency f 1 , variable over this band, are modulated by a lower frequency f 0  and are passed, together with oscillations of the lower frequency f o  through the circuit. From the output of the circuit the modulated oscillations are separated from those of the lower frequency f o  and are demodulated, the phase difference of the two oscillations of frequency f o  being measured as the frequeney f 1  is varied over the frequency band, one of these two oscillations of frequency f 0  having been passed through the circuit directly and the other as a modulation of the variable frequency oscillations. A crystal-controlled oscillator 1, Fig. 1, generates oscillations having a frequency f 0 , which is much lower than the lowest frequency of the band over which measurements are to be made, and has two parallel outputs. One of these is connected to a modulator 2, to which is also connected a calibrated oscillator 3 of variable frequency f 1 . The other output is connected to an amplifier 4, the output of which is connected, together with the output of the modulator 2, through an amplifier 5 to the input terminal 6 of the circuit 7 under test. The output terminals 8 are connected through an amplifier 9 to a series circuit 10 resonant at frequency f o . This circuit separates the oscillations of frequency f 0  from those of frequency f 1  modulated at f 0 , since only the oscillations of frequency f 0  appear across either the condenser or the inductance and only the modulated oscillations of frequency f 1  appear across the complete resonant circuit 10. The terminal 11 of the series circuit 10 is connected through a filter 13, which selects oscillations of frequency f o , in series with an amplifier 14 to a time base generator 15, the output of which is connected to the X-plates 16 of a cathode-ray tube 17. The terminal 12 of the circuit 10 is connected to a demodulator 18 which derives the oscillation of frequency f 0  from the modulated oscillations of frequency f 1 . The output of the demodulator 18 is connected through a filter 19 which selects the oscillations of frequency f 0 , an amplifier 20, and a variable phase shifter 21 to the input of a pulse generator 22, the output of which is applied to the Y-plates 23 of the cathode-ray tube 17. The pulse generator 22 operates at one of the points in each cycle at which the amplitude of the oscillation of frequency f o  passes through its mean value. Fig. 3 shows a circuit for ensuring the desired timing of the pulse with respect to the voltage oscillation, which is applied between a terminal 26 and earth. The terminal 26 is connected through a condenser 27 and resistance 28 to the anode of a diode 29 and the cathode of a diode 30. A potentiometer chain comprising resistors 33, 34, 35 is connected between the positive H.T. terminal 38 and earth, the central resistance 34 having a variable tapping which is connected through a resistor 32 to the junction of the diodes 29, 30. The cathode of the diode 29 and the anode of the diode 30 are connected to the ends of the resistor 34. The values of the resistors 33, 34, 35 are so chosen that the potential between the tapping point and either terminal of the resistor 34 is less than half the peak potential of the least voltage oscillation to be applied to the circuit. The diodes 29, 30 then pass current when the amplitude of the oscillation passes above or below the potentials of the points in the potentiometer chain to which they are connected and limit the voltage oscillation at these potentials. The output from the terminal 39 to earth is a symmetrical trapezoidal voltage wave, the amplitude of which remains constant regardless of the amplitude of the input voltage, and the midpoints of the sides of which remain fixed in time with respect to the input voltage oscillation. The trapezoidal voltage output is applied to the pulse generator 22, Fig. 1, which is operated by a very limited portion of the voltage range of the trapezoidal wave near the mid-points of the sides. The output pulses appear as a " pip " on the scan of the cathoderay tube 17, which " pip " may be moved, at will, along the scan by adjusting the phase shifter 21. The cathode-ray tube is provided with a scale which has a central zero and is graduated in time units. The frequency f 1  is set at one end of the band and the phase shifter 21 adjusted until the " pip " coincides with the scale zero. The frequency f 1  is then varied through the band and the movement of the " pip " over the scale is recorded in terms of time. Specification 628,615 is referred to.